1. Field of the Invention
The invention relates to lasers.
2. Discussion of the Related Art
In optical communications, the use of wavelength division multiplexing (WDM) to enable high bandwidth communication is expanding. A WDM system simultaneously transmits several distinct communications over one fiber by transmitting the communications in different wavelength channels. The wavelength channels are typically closely spaced to more efficiently use wavelength ranges for which the optical fiber has better transmission properties, e.g., lower attenuation.
The need for low cost and closely spaced wavelength channels poses challenges to the development of WDM systems. In particular, WDM transmitters have to produce output wavelengths that do not drift between the different wavelength channels. Unfortunately, the output wavelengths of the semiconductor lasers typically used in WDM transmitters tend to drift unless complex and expensive temperature controls are added. The drift results from temperature variations in laser cavities that lead to changes in the optical properties of the semiconductor medium therein.
Another problem in WDMs relates to the need for multi-chromatic transmitters. Such transmitters are able to transmit light in multiple wavelength channels of the WDM system concurrently. Disadvantageously, multi-chromatic transmitters are usually more complex and high cost.
A multi-chromatic laser embodying the principles of the invention features multiple laser cavities that share a common gain medium. The laser may include several wavelength-selective reflectors with different characteristic reflection wavelengths as an advantageous way of enabling the multiple cavities to share the common gain medium. Such a laser can be used, for example, as a multi-chromatic light source for an optical transmitter in a WDM or other optical network. Such transmitters are typically less complex than prior art multi-chromatic optical transmitters.
One embodiment of the invention features a so-called external cavity laser in which the laser cavity includes an internal waveguide and an external waveguide that is serially connected to the internal waveguide. The external waveguide has cascaded first and second wavelength-selective reflectors for reflecting first and second wavelengths, respectively. The first and second reflectors define the multiple laser cavities.